The present invention relates to a semiconductor laser device made of AlGaInP or its analogues and a method for fabricating the same. In particular the present invention relates to a semiconductor laser device made by utilizing MBE process which may grow an AlGaInP-based layer at a lower temperature to provide an improved crystal quality, a lower threshold in oscillation and a high efficiency in light emission and a method for fabricating the same.
AlGaInP-based semiconductor laser devices are used as optical sources for optical disc systems, laser printers, bar-code readers and the like, accordingly researches and developments for such devices have been enthusiastically carried out. There are some prior patents relating to AlGaInP semiconductor laser devices and a method for fabricating the same, such as Japanese Laid-Open Patent Publication No. 8-228041, Japanese Laid-Open Patent Publication No. 8-228047, Japanese Laid-Open Patent Publication No. 6-296062, Japanese Laid-Open Patent Publication No. 2-168690, Japanese Laid-Open Patent Publication No. 8-181385, Japanese Laid-Open Patent Publication No. 5-67839, Japanese Laid-Open Patent Publication No.7-50453, Japanese Laid-Open Patent Publication No. 7-50452, Japanese Laid-Open Patent Publication No. 7-22696 and Japanese Laid-Open Patent Publication No. 6-275915.
In the above patents, MOCVD process has been mainly used for growing AlGaInP crystal layers which are utilized to construct AlGaInP-based semiconductor laser devices, since the MOCVD process has provided better crystal qualities than MBE process which is one of major crystal growth processes. While the MBE process provides a higher carrier density in p-type semiconductor layers where the carrier density is important to improve electric characteristics in semiconductor laser devices and utilizes Be of a lower diffusion as an impurity to achieve such carrier density so as to realize a semiconductor laser devise of long-term reliability. There are two major advantages of the MBE process. For these reasons, an improvement in crystal qualities for crystals grown by the MBE process is effective to obtain an improved characteristic of AlGaInP-based semiconductor laser devices.
Now, AlGaInP-based semiconductor laser devices by MBE (molecular beam epitaxy) process will be described hereinafter.
FIGS. 1 to 4 are respectively a part of a flow chart illustrating a conventional process for fabricating AlGaInP semiconductor laser devices. FIGS. 5 to 7 also are respectively a part of a flow chart continuing from FIGS. 1 to 4 for illustrating the conventional process.
As shown in FIG. 1, an n-type (Al0.72Ga0.28)0.51In0.49 cladding layer 22, a Ga0.51In0.49P active layer 23, a first p-type (Al0.72Ga0.28)0.51In0.49P cladding layer 24, a non-dope Ga0.62In0.38P etch-stop layer 25, a second p-type (Al0.72Ga0.28)0.51In0.49P cladding layer 26, a p-type Ga0.51In0.49P intermediate layer 27 and a p-type GaAs cap layer 28 are successively grown by the MBE process at a temperature of 450xc2x0 C. on amain facet of an n-type GaAs substrate 21 which has a facet-direction (100) just aligned in place. Then an Al2O3 layer 29 is deposited on the cap layer 28.
Then, a resist layer 30 is applied onto the Al2O3 layer 29 for photoetching to obtain stripe-shaped Al2O3 layer 29 by a pattern process. As shown in FIG. 2, an etching process follows using the Al2O3 layer 29 as a mask to partially remove the cap layer 28, the p-type Ga0.51In0.49 intermediate layer 27 and the second p-type (Al0.72Ga0.28)0.51In 0.49P cladding layer 26 so as to form a ridge beneath the Al2O3 layer. Then, as shown in FIG. 3, after removing the resist layer 30, a second MBE process is carried out to form an n-type GaAs current blocking layer 31 at the both sides of the ridge.
During the second MBE process, a GaAs crystal 32 of a polycrystalline is grown on the surface of the Al2O3 layer 29. The a resist layer 33 is applied by a spinner wherein the resist layer 33 is not substantially applied on the polycrystalline GaAs crystal 32 but on the current blocking layer 31. Subsequently, the resist 33 on the whole surface is ashed by O3-UV to have the resist 33 coated only on the n-type GaAs current blocking layer 31 as shown in FIG. 4.
Then, as shown in FIG. 5, the polycrystalline GaAs crystal 32 is removed by etching by using the resist 33 as a mask. Subsequently, the resist 33 is removed and the Al2O3 layer 29 is also removed by etching as shown in FIG. 6. Next, a third MBE process is carried out to form a contact layer 34. Finally electrodes 35 and 36 are formed respectively on the top of the array obtained as described above and on the back of the n-type GaAs substrate 21 to obtain an AlGaInP-based semiconductor red-laser device as shown in FIG. 7.
Since MBE process supplies metal material in molecules, it allows the metal to grow at a lower temperature comparing with an MOCVD (Metal Organic Chemical Vapor Deposition which is also a type of chemical vapor deposition) where a material is supplied in an organic metal. Further, the MOCVD process allows the metal to grow only at a temperature ranging from 600xc2x0 C. to 700xc2x0 C. which is higher than the decomposition temperature of the organic metal. In other words, the growth temperature must be higher than 520xc2x0 C. which is the evaporating temperature of In atoms, whereby the thickness of the grown crystal becomes smaller comparing with the supplied In. That is to say, the crystal is grown during re-evaporation of itself. Once re-evaporation of the crystal happens, its compound crystal rate is varied not only by material supply but also by its growth temperature, thereby making it difficult to control characteristics of a semiconductor laser device, such as oscillating wave-length.
However, even by using MBE process it is still preferable to grow a crystal at a higher temperature in order to improve crystal qualities. If an AlGaInP-based material is grown at a temperature lower than 400xc2x0C., for example, a specific resistance of the crystal is too large to fabricate a semiconductor laser device because metal molecules of the material do not seem to locate in place, resulting in a poor crystal quality.
While, a substrate having an aligned facet-direction of (100) obtained by MBE growth process at a higher than 480xc2x0 C. gives a wide spectrum of photoluminescence (referred to as PL hereinafter) which is unfavorable for a crystal applied to a semiconductor laser device.
Further, an AlGaInP-based semiconductor layer grown in the conventional manner tends to be affected by impurities on a GaAs substrate surface, resulting in a poor morphology, causing a crystal defect and the like.
An object of the present invention is to provide a semiconductor laser device having a lower oscillation threshold and a high light-emission efficiency by growing AlGaInP-based semiconductor layers on a GaAs substrate having a facet, which is to be a main facet, inclined by xcex8xc2x0 in [011] direction from (100) facet, that is to say, by growing AlGaInP semiconductor layers of a good crystal quality.
Another object of the present invention is to provide a semiconductor fabricating method which permits growing at a lower temperature than the re-evaporation temperature of In, thus improving the crystal quality of AlGaInP-based semiconductor device by MBE growth process allowing a small deviation in compound crystal ratios to give a stable characteristic and which has less influence by impurities on a GaAs substrate surface, thereby giving a good morphology when growing an AlGaInP-based semiconductor layer to lessen crystal defects.
Still another object of the present invention is to provide a semiconductor laser device having a cladding layer of bandgap Egc made of III-V group compound semiconductor layer and an active layer of bandgap Ega which are stacked on a substrate having a facet, which is to be a main facet, inclined by xcex8 in [011] direction from (100) facet so as to form a ridge stripe wherein the relationship between the bandgaps Ega and Egc is represented by Ega less than Egc and the extending direction of the ridge stripe is [01-1].
Further, another object of the present invention is to provide a semiconductor laser device wherein the angle xcex8xc2x0 inclined in [011] direction from (100) facet is between 7xc2x0 and 15xc2x0.
Another object of the present invention is to provide a semiconductor laser device wherein a cross-sectional shape of the ridge stripe is asymmetric along an axis extending in the stacked direction of the III-V group compound semiconductor layers in the cross section.
Another object of the present invention is to provide a semiconductor laser device wherein a cross-section shape of the ridge stripe has a larger width which is closer to the active layer and a smaller width which is farther from the active layer and acute angles out of angles formed by a stack facet and both sides of the ridge stripe respectively are 54.7xc2x0xc2x1xcex8xc2x0.
Still another object of the present invention is to provide a semiconductor device wherein the active layer is a GaInP/AlGaInP multiple quantum well and the cladding layer is AlGaInP-based.
Another object of the present invention is to provide a semiconductor laser device wherein the substrate is GaAs-based and has a buffer layer made of GaAs and formed thereon.
Another object of the present invention is to provide a semiconductor laser device wherein the substrate is GaAs-based and has a buffer layer made of GaAs and another buffer made of GaInP and formed thereon.
Still another object of the present invention is to provide a semiconductor laser device including the steps of stacking a cladding layer of bandgap Egc made of III-V group compound semiconductor layer and an active layer of bandgap Ega on a substrate having a facet, which is to be a main facet, inclined by xcex8xc2x0 in [011] direction from (100) facet that formed a ridge stripe wherein the method comprises the steps of extending the ridge stripe in [01-1] direction and chemically fabricating the ridge stripe by chemically etching.
Another object of the present invention is to provide a semiconductor laser device wherein III-V group compound semiconductor layers including the cladding layer and the active layer are formed by MBE process.
Another object of the present invention is to provide a semiconductor laser device wherein the growth temperature of GaInP, AlGaInP semiconductor layers by the MBE process is between 400xc2x0 C. and 520xc2x0 C.